Functional dyes used as an indicator in aerobically hardening compositions

ABSTRACT

The invention relates to the addition of dyes to aerobically curable adhesive compositions based on free-radically polymerizable compounds. The dyes serve as user-friendly indicators for monitoring the oxygen intake required to trigger the polymerization. The indicator dyes are especially suitable for aerobically adhesive compositions which contain activator systems for forming peroxides upon contact with air. Suitable dyes and indicators are those whose color change in aqueous solution lies within a pH range from 3.0 to 6.5.

The present invention relates to an aerobically curing reactive adhesivecomposition and to its preparation and use.

Adhesive bonds are encountered in the domestic appliance, electrical,electronics, furniture, sports goods, construction (interior andexterior), ceramics and automotive industries, including their supplieroperations. The mechanical engineering sector profits from the adhesiveproperties in the same way as does valve or installation engineering,where additionally there are sealing functions to be undertaken. Inaddition to same-kind material combinations of steel, aluminum, brass,copper, coated metal surfaces, wood, glass, PVC, PC (polycarbonate), andABS, pairings of different materials are of particular importance fromthe standpoint of adhesives technology. In the case of these compositebonds, differences in thermal expansion behavior, or loss of adhesion,which occurs inter alia on materials such as ferrite or ceramic, oftenmean that the performance limit of known adhesive systems is reached. Incertain applications, the adhesives are further required to possessdamping properties, and/or in the case of different electrochemicalpotentials must not have any corrosion-promoting properties.

Known adhesive systems such as 2-part epoxy resins, cyanoacrylates,dispersion adhesives, contact adhesives, 2-part acrylate adhesives,2-part poly-urethane adhesives or anaerobically curing adhesives areknown to have performance limits in terms of gap-filling capacity,elasticity, automatic metering, heat stability, volatility, and aging.

Furthermore, for the bonding of a very wide variety of materials in thecontext of automated industrial manufacture, the demand is predominantlyfor short cycle times. Aerobically curable adhesives, as are known, forexample, from DE-A-4000776 or DE-A-4123194, meet many of theabovementioned requirements. As is known, aerobically curable adhesivecompositions are free-radically polymerizable multi-substance mixtureswhich are stable on storage to the exclusion of air, especially to theexclusion of oxygen. Furthermore, these compositions are required tocontain no peroxides. For this purpose, as is known, these compositionsare thoroughly degassed, and/or a deoxygenator is added to them. Thisenables a controlled initiation of polymerization: for example,adhesives and sealing, coating and molding compounds are initiallystable on storage for a period of several months, which is sufficientfor practical purposes, and then cure simply within a short time even atroom temperature when air is admitted. Activation of the aerobicadhesive requires waiting times, before or after joining, during whichthe oxygen of the air must have access to the adhesive composition. Forautomatic industrial manufacture with its short cycle times,DE-A-4441414 proposes apparatus and measures for the metering andstorage of aerobic adhesives. According to the teaching of the document,the time between application of the adhesive and joining of the parts tobe connected may also be reduced by adding the oxygen to the adhesiveeven before the latter is applied. To this end, the application devicecomprises a mixing chamber in which the oxygen—whether in pure form oras air—is added to the adhesive under greater or lesser pressure.

For activation, one-component aerobically curing reactive adhesivesrequire a certain time in order to take up atmospheric oxygen (aircontact time). The air contact time required is dependent on theadditives and stabilizers used in the adhesive composition, ontemperature and on other parameters, so that for the user it is firstnecessary to determine the particular optimum air contact time by meansof experiments. For unproblematic and low-error application of suchadhesives, therefore, there is a need to be able to determine, simply,the sufficient uptake of oxygen by the aerobically curable composition.

The solution provided by the invention to this problem is given in theclaims. It consists essentially in the provision of aerobically curablecompositions which are based on free-radically polymerizable compoundsand comprise an activator system for forming peroxides and a dye as anindicator of sufficient oxygen uptake. Furthermore, the aerobicallycuring compositions of the invention may comprise further customaryadditives.

The addition of dyes to adhesive compositions for the purpose ofmonitoring the degree of curing is already known. For instance,EP-A-342965 describes an adhesive composition comprising a one-componentheat-curing adhesive and an effective fraction of a water-insolubleindicator which does not react with the components of the adhesive atroom temperature but which on heat curing produces a color change toenable a visible determination of the state of cure of the adhesive.

EP-A-79703 describes a two-component adhesive composition whose firstcomponent comprises a vinyl monomer and a polymerization initiator andwhose second component comprises a polymerization accelerator, andadditionally a redox indicator having an E_(o) of greater than +0.01 toless than +0.76 volt and an R_(H) of from 13.5 to 28. According to theteaching of this document, the dye serves to indicate the time fromwhich the mixture of the two-component adhesive composition, aftermixing, should no longer be used (end of pot life). This dye furtherserves to distinguish the two reactive components and to ascertain thecompleteness of mixing.

DE-A-4001417 describes a toughened two-component epoxy resin adhesivebased on an epoxy resin and on an amine hardener, the resin componentcomprising a color indicator which indicates the homogenous mixing ofresin and hardener by means of a change in color.

DE-A-4132726 and DE-A-4112649 describe adhesive systems based on epoxyresins to which a color or, respectively, a color pigment is admixedwhich changes its color on curing, so making it possible to determinecomplete curing of the adhesive composition.

EP-A-96500 describes a two-component adhesive composition based onpolymerizable vinyl monomers, free-radical stabilizers, and afree-radical curing system. When the two components are mixed a colorchange takes place which allows for complete mixing and represents avisible indicator of the limited pot life. Additionally, the curingprocess is accompanied by a color change.

None of the abovementioned documents discloses an indicator for thesufficient oxygen uptake of aerobically curable adhesive compositions.

The free-radically polymerizable compounds of the present inventioncomprise

A) at least one polyurethane (meth)acrylate of the general formula

[H₂C═CR¹—C(═O)—O—R²—O—C(═O)—NH—]_(n)R³  (I)

 in which

R¹=hydrogen or a methyl group

R²=a linear or branched alkyl group having 2 to 6 carbon atoms oralkylene oxides having 4 to 21 carbon atoms,

n=2 or 3, and

R³ if n=2 is:

 [—Q—NH—C(═O)]₂[{—O—R⁴—O—C(═O)—NH—Q′—NH—C(═O) }_(m)—O—R⁴—O—]

 in which m=0 to 10,

R⁴ is

a) a polycaprolactonediol radical,

b) a polytetrahydrofurfuryldiol radical or

c) a diol radical derived from a polyester diol characterized by a C:Oratio of >2.6, a C:H ratio of <10, and a molecular weight from 1000 to20,000, or

d) a diol radical derived from a polyester diol which is liquid at 20°C. and has a molecular weight of from 4000 to 10,000, measured with theaid of GPC (gel permeation chromatography), and

R³ if n=3 is:

[—Q—NH—C(═O)—O—((CH₂)₅—C(═O)_(p)—]₃R⁵

 in which R⁵ is a triol radical of a linear or branched trivalentalcohol containing 3 to 6 carbon atoms and p is from 1 to 10, and

Q and Q′, independently of one another, are aromatic, aliphatic orcycloaliphatic groups which contain 6 to 18 carbon atoms and are derivedfrom diisocyanates or mixtures of diisocyanates, and also

B) one or more (meth)acrylate comonomers, where said compositioncomprises 20-80% by weight of the polyurethane (meth)acrylate, and80-20% by weight of the (meth)acrylate comonomer, based on the overallamount of polymerizable compounds, and the activator system is ahydrazone activator system.

The compounds of the above formula (I) can be prepared in accordancewith processes known per se in the prior art by reacting an acrylate(R¹=H) or methacrylate (R¹=CH₃) containing hydroxyl groups in the estergroup with compounds containing isocyanate groups, to form urethanegroups.

In accordance with the invention, said acrylates or methacrylatescomprise hydroxyalkyl acrylates or methacrylates, in which the alkylgroups may be linear or branched and contain between 2 and 6 carbonatoms. According to the invention, it is also possible to use the estersof acrylic acid or methacrylic acid with polyethylene glycol and/orpolypropylene glycol. Such acrylates or methacrylates contain 4 to 21carbon atoms in the ester group, corresponding to from 2 to 10 ethyleneoxide units and from 1 to 7 propylene oxide units. The preparation ofsuch esters is known to the skilled worker.

Suitable acrylates or methacrylates are those for which R² contains anethylene, propylene, isopropylene, n-butylene or isobutylene group orethylene oxide or propylene oxide units.

For the reaction of the isocyanates with the hydroxyl-bearing acrylatesor methacrylates, however, preference is given to hydroxyethyl acrylate,hydroxyethyl methacrylate, 2-hydroxypropyl acrylate, 2-hydroxypropylmethacrylate, 3-hydroxypropyl acrylate, 3-hydroxypropyl methacrylate,6-hydroxyhexyl acrylate, 6-hydroxyhexyl methacrylate, polyethyleneglycol acrylate, polyethylene glycol methacrylate, polypropylene glycolacrylate, and polypropylene glycol methacrylate.

Where n=2, the compounds of the above general formula (I) are reactionproducts of the abovementioned hydroxyl-containing acrylates ormethacrylates with isocyanates obtainable by reacting appropriate diolswith diisocyanates. Said diols comprise a) polycaprolactonediols, b)polytetrahydrofurfuryldiols, and c) specific polyesterdiols. The molarratio in the reaction of the diols with the diisocyanates may vary from1:2 to 1:1.1.

For further details, especially as regards the detailed structure of thecompounds of the general formula (I) and of the diols used in accordancewith a), b) and c), reference is made to DE-A-4441414 page 3 lines to 60to page 5 line 5, which are expressly part of the present invention.

The adhesive composition may further comprise one or more acrylate ormethacrylate comonomers. These monomers are selected from allylacrylate, allyl methacrylate, methyl acrylate, methyl methacrylate,2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2- or3-hydroxypropyl acrylate, 2- or 3-hydroxypropyl methacrylate,6-hydroxyhexyl acrylate, 6-hydroxyhexyl methacrylate, benzoylmethacrylate, phenylethyl methacrylate, 2-phenoxyethyl methacrylate,morpholinoethyl methacrylate, glycidyl methacrylate,piperidylacrylamide, neopentyl methacrylate, cyclohexyl methacrylate,tert-butyl methacrylate, and tetrahydrofurfuryl methacrylate, ormixtures thereof. It is also possible to use dimethacrylates, such asespecially TEGDMA (tetraethylene glycol dimethacrylate).

The composition of the invention preferably contains from 20 to 70% byweight of the polyurethane (meth)acrylate and from 80 to 30% by weightof the (meth)acrylate comonomer, based on the overall amount ofpolymerizable compounds.

The adhesive compositions of the invention are used as free-radicallypolymerizable one-component systems. In each case, however, an initiatoror activator system is used which triggers the polymerization ofolefinically unsaturated systems. A feature of such activators is thecapacity to be activated by the ingress of ambient air, this activationbeing intended—in preferred embodiments—to ensue even at roomtemperature. Activator systems of this kind consist in general of aninitiator component and an accelerator component.

One-component adhesive systems should be stable on storage, shapeable intheir application form, in particular paste-like to fluid, and shouldhave a pot life sufficient for processing but should then cure to adimensionally stable form by the simple action of ambient air. Theone-component adhesive systems must be kept protected from the ingressof air up until the time they are used.

For the purposes of the invention, hydrazones have been used asinitiators or main components in activator systems for the curing of theunsaturated compounds by means of ambient air. For acceleration, thehydrazone compounds are preferably used with metal compounds at leastpartially soluble in the system.

The hydrazone components used with preference in accordance with theinvention are of the following general formula II

The radicals R⁶, R⁷ and R⁸ from this general formula (II) have thefollowing meanings:

R⁶ is a straight-chain, branched or cyclic and optionally substitutedalkyl radical or an optionally substituted aryl radical.

The definition of R⁷ can be the same as or different than that of theradical R⁶, R⁷ being hydrogen, a straight-chain, branched or cyclic andsubstituted alkyl radical, or an optionally substituted aryl radical.

In one particular embodiment, the radicals R⁶ and R⁷, together with theshared substituted carbon atom, may also form a cycloaliphatic radical,which may also be substituted.

The radical R⁸ is again a branched, straight-chain or cyclic andoptionally substituted alkyl radical or an optionally substituted arylradical.

The radicals R⁶, R⁷ and R⁸ may all, or at least in part, be the same;however, each of these radicals may also differ in its structure fromthe two other radicals. Suitable alkyl radicals are especiallystraight-chain and/or branched alkyl radicals having up to 40 carbonatoms, preferably up to about 20 carbon atoms. The minimum carbon numberof cyclic radicals is determined by the stability of the respective ringsystem. A preferred lower limit in this respect is 5 and, in particular,6 carbon atoms.

The radicals R⁶, R⁷ and R⁸ may be unsubstituted or substituted. Itshould be borne in mind here that by substituting these radicals, andespecially the radical R⁸, it is possible to exert a certain influenceover the reactivity of the activator system. If, for example, anaromatic system, especially a phenyl radical, is present as the radicalR⁸, then by appropriate substitution of this phenyl radical it ispossible to accelerate the reaction by providing known electron donatingsubstituents on this phenyl radical in R⁸. Electron donating and thusreaction accelerating substituents are, for example, alkyl, alkoxyand/or halogen substituents on the phenyl radical in R⁸; conversely,strongly electron withdrawing substituents, of the type of the —NO₂group, have a retarding effect on the progress of reaction. In relationto such electron donating and electron withdrawing effects bysubstitution, and on the structure of the respective substituents, thereexists extensive general technical knowledge, to which reference is madehere.

The radicals R⁶ and R⁷ from the general formula II may also act toinfluence the rates of the progress of the reaction, through the choiceof their respective constitution and of any substitution provided.

In relation to the extraordinary diversity of the particular nature ofthe radicals R⁶, R⁷ and R⁸ from the compounds of the general formula II,information is given by U.S. Pat. No. 4,010,152 with its formulae fromcolumns 6 to 14. The hydrazone compounds are shown here already in theform of their hydroperoxides, as are also formed in accordance with theinvention, in situ on the ingress of atmospheric oxygen in the course ofthe reaction. The extensive details of this last-mentioned document inrelation to the particular nature of the hydrazone compounds and on thehydroperoxides formed from them in situ apply to the teaching of theinvention in connection with the definition of the compounds of thegeneral formula II.

Specific examples of hydrazones to be used are acetophenonemethylhydrazone (R⁶=phenyl, R⁷=R⁸=CH₃) and acetophenone t-butylhydrazone(R⁶=phenyl, R⁷=CH₃, R⁸=C(CH₃)₃).

For further details regarding the preparation and mode of action of thehydrazones, reference is made to WO-A-91/10687.

In addition, a proportion of soluble metal compounds may be present inthe system.

In this context, metal compounds employed are in particular those ofmetals able to exist in a plurality of valence states (dryers). Selectedrepresentatives of the transition metals may be particularly activehere. Irrespective selection of the metal may inter alia be accorded arate-determining character in relation to initiating polymerization.Components highly active at room temperature are derived, for example,from copper, cobalt, vanadium and/or manganese. Compounds of iron,however, are accorded particular significance and good acceleration ofthe action.

Compounds suitable for operating at room temperature, which isparticularly advantageous for many fields of application, are especiallyiron, cobalt and/or manganese compounds, alone or in a blend withfurther metallic components such as compounds of lead, cerium, calcium,barium, zinc and/or zirconium. Reference may be made here to therelevant technical literature. A comprehensive representation of suchsystems can be found, for example, in “Ullmann, Enzyklopädie dertechnischen Chemie”, 4th edition, Volume 23 (1983), pages 421 to 424.

The metals concerned here are used in the form of compounds such thatthey are at least partially soluble in the overall system. Suitabletypes in this context include not only soaplike metal compounds but alsoother forms, especially those attached in complex form to organicradicals. Typical examples for operating in accordance with theinvention include the use of corresponding metal naphthenates and/ormetal acetylacetonates. Where the solubility of inorganic salts in thesystem is sufficient, however, the use of inorganic systems of this kindis also possible. One typical example of this is iron chloride, whichwhen used in the system of the invention exhibits a markedlyaccelerating action.

It may be judicious to use the metal compounds in each case in a lowvalence state of the metal—i.e., for example, as cobalt(II) ormanganese(II). In other cases it is also suitable to use the metalcompound in the higher valence state of the metal. For example, iron mayalso be used in the form of the Fe³⁺ compounds.

Additionally, the compositions of the invention may comprise reducingagents having an accelerator effect, as are used in customary redoxsystems for initiating polymerization. Reference is made to the relevantextensive literature, examples being W. Kern, makromol. Chem. 1, 249(1947), and C. Sma, Angew. Makromol. Chem. 9, 165 (1969), and also tothe general technical knowledge, as described, for example, in HoubenWeyl “Methoden der organischen Chemie” Volume 14/1, 263 to 297.

A class of compound which has been found particularly active here isthat of the alpha-hydroxy ketones, represented for example by butyroin,benzoin or acetoin, although the teaching of the invention is by nomeans restricted thereto. Reference is made to the modifications shownin the examples of the invention. The essential point is that, althoughthis class of substance is able to undertake an importantreaction-accelerating function in the activated systems used inaccordance with the invention, its use is, however, not mandatory.

Initiators or activator systems comprising the principal componentsdepicted here—hydrazone compound, at least partially soluble metallicdryer and/or, if desired, additional accelerator—are able, followinguptake of ambient air, to initiate all of the free-radicallypolymerizable, olefinically unsaturated systems used to date with themost diverse initiator systems, especially peroxidic initiator systems.Regarding definition of the further constituents of the shapeable,free-radically polymerizable multi-substance mixtures described inaccordance with the invention it is possible to this extent to refer tothe extensive information in the relevant literature dealing with thepreparation and processing of the systems based on olefinicallyunsaturated, free-radically polymerizable compounds. In the art todaythere is an extremely large number of systems of substances of thisnature which are tailored in terms of their properties to the particularend use. Olefinically unsaturated reactive systems may be based onselected free-radically polymerizable, ethylenically unsaturatedcompounds, or on mixtures of two or more types of such compounds. Theseethylenically unsaturated polymerizable components may be monofunctionalcompounds and/or poly-functional reactive components.

As an indicator of sufficient oxygen uptake by the aerobically curablecompositions of the invention, use is made of dyes which are known perse. The selection criterion for these dyes is a clearly visible colorchange, which must correlate with the sufficient uptake of oxygen by theadhesive composition. Color change is understood to mean the developmentof a color which differs from the original state, or an increase ordecrease in the intensity of the original color. Furthermore, theaddition of the dyes in the concentration ranges intended for use mustnot adversely affect the physical properties of the curable system orits setting properties.

Leuco dyes such as leuco crystal violet and leuco malachite green aresensitive to oxidation and exhibit a marked color change afteroxidation. Although investigations of these leuco dyes in the adhesivecompositions of the invention did show an alteration in color as afunction of the period of contact with the air, this alteration in colordid not correlate with the degree of polymerization.

In accordance with the invention, suitable dyes and indicators are thosewhose color change in aqueous solution lies within a pH range from 3.0to 6.5. Specific examples of such dyes are bromophenol blue,bromochlorophenol blue, bromocresol green, 5-nitro-indazolinone,alizarin S, methyl red and ethyl red. The amount of the dyes to be useddepends on the end use and on the inherent color of the adhesivecomposition without the dye. In each specific case it can be determinedsimply by means of expert considerations and/or by means of preliminaryexperiments. In very general terms, the weight fraction of dyes is notmore than 5% by weight, preferably from 0.01 to 2% by weight based onthe overall composition.

The purity of the composition for the target polymerization, inparticular for storage, is accorded particular importance. In accordancewith the invention, the oxygen may be removed by devolatilization. Forthis purpose, preferably, the homogenized composition minus thehydrazone is exposed to a vacuum of 0.5 torr at approximately −196° C.for from 2 to 5 minutes and is then warmed to room temperature.Repeating this freeze/thaw cycle several times removes residual oxygenfrom the system. A simple and practical measure is to store thecomposition at 80° C. in the absence of air. No polymerization must haveoccurred after 24 hours. Devolatilization can also be carried out byother means; for example, with the aid of ultrasound, or chemically.

It is known that olefins and other unsaturated compounds, especiallypolymerizable monomers, must be stabilized for the purpose of theirstorage. The procedure in this case starts from the following basictypes of reaction:

1. stabilization to O₂ by adding antioxidants, and

2. stabilization to free radicals by adding free-radical inhibitors.

From the general chemical knowledge, a listing may be given below oftypical stabilizer components, the mechanism to which the stabilizer isto be assigned being indicated in brackets after the specific compoundin each case: pyrogallol (1), O₂-inhibited acrylates (1), hydroquinone(1,2), hydroquinone monomethyl ether (1,2), butylated hydroxytoluene(2), and phenothiazine (2). Of particular importance for stabilizing thesystem against unwanted premature reaction are deoxygenators of thetriphenylphosphine type, and also iodide-iodine solutions. The amountthereof depends on the end use. In each specific case it can bedetermined simply by means of expert considerations and/or by means ofpreliminary experiments. In very general terms, the weight fraction ofstabilizers is not more than 5% by weight, preferably from 0.01 to 1% byweight, based on the overall composition. Too high an amount retards thedesired polymerization on ingress of air.

The compositions of the invention are preferably used as aerobicallycuring sealing materials or adhesives but may also be used as aspreadable composition for surface coatings, as a molding compound, oras an additive to inks, including printing inks. For this purpose it maybe necessary to use not only the constituents already mentioned but alsosoluble and/or insoluble fillers, elasticizers, thickeners, thixotropicagents, pigments, adhesion promoters and the like, without threateningthe ability of the initiator system of the invention to function. Theprecondition for this is, of course, that the selection of auxiliariesand fillers rules out disruptions to the interaction of the initiatorcomponents. In this respect, general chemical knowledge applies.

The compositions of the invention contain (based on the overallcomposition):

at least 20, preferably at least 40% by weight of free-radicallypolymerizable compounds having an olefinically unsaturated double bond,

at least 0.1, preferably from 0.5 to 7.5 and, in particular, from 1 to4% by weight of a hydrazone compound,

from 0 to 5, preferably from 0.01 to 1% by weight of stabilizer,

from 0 to 5, preferably from 0.005 to 1 and, in particular, from 0.1 to0.5% by weight of a metal compound,

from 0 to 80, preferably from 10 to 50% by weight of auxiliariesdepending on the use of the composition, e.g., fillers, pigments,thickeners, adhesion promoters, and elasticizers, and also

from 0.01 to 5% by weight of an indicator dye.

The examples which follow serve as specific elucidation of preferredembodiments of the invention, the selection of the examples not beingintended to constitute any restriction on the scope of the subjectmatter of the invention.

EXAMPLES

An aerobic adhesive composition was prepared from 40% by weight ofbenzyl methacrylate, 12% by weight of hydroxypropyl methacrylate and 48%by weight of polyurethane methacrylate comprising a polycaprolactonetriol and a polycaprolactonediol, and also from toluene diisocyanate andhydroxypropyl methacrylate, preparation taking place as described inExample V 1) on page 10 of DE 44 41 414 A1. The adhesive compositionlikewise comprised the additions of acetophenone tert-butylhydrazone asinitiator, as specified on page 11 of that document, and also furthercustomary additives, in particular the abovementioned stabilizers.

50 mg of bromochlorophenol blue, as dye, were dissolved in 250 g of thisadhesive composition.

The composition was investigated for its adhesive properties in thecontext of bonding to a wide variety of substrates (DIN 53281, 53282 and53283). To this end, 5 drops of the adhesive composition (approximately60 mg) were applied to one side of the substrate in question, andjoining was carried out following an air contact time of 5 minutes.After this air contact time, a color change from yellowish orange (theoriginal color of the adhesive composition) to green was observed.

The table below lists the strength values of the adhesive bond obtained,after the air contact time indicated above. These values are mean valuesdetermined from at least 5 measurements. “CF” denotes cohesive fracturein the adhesive joint, “MF” denotes material fracture of the substrate.The times indicated for storage refer to the storage time of the bondedsubstrates prior to the strength test in question.

Tensile shear Force strength [N] [N/mm²] Through- (mean) (mean) Fracturecuring Alu.: sandblasted 4164 16.7 CF satis. (after 72 h storage)Standard deviation: ±314.1 ±1.3 Steel: sandblasted 6582 26.3 CF satis.(after 3 h 80° C. - storage) Standard deviation: ±136.0 ±0.5 Steel:sandblasted 5734 22.9 CF satis. (after 72 h - storage) Standarddeviation: ±405.1 ±1.6 Steel: sandblasted 5878 23.5 CF satis. (after 72h + 3 h 80° C. - storage) Standard deviation: ±306.4 ±1.2 ABS: smooth2390 9.6 MF (after 72 h - storage) Standard deviation: ±26.1 ±0.1 PVC:smooth 2236 8.9 MF (after 72 h - storage) Standard deviation: ±507.3±2.0 PC: smooth 2198 8.8 MF (after 72 h - storage) Standard deviation:±583.4 ±2.3 Alu.: sandblasted 4574 18.3 CF satis. (after 3 h 80° C. -storage) Standard deviation: ±171.7 ±0.7 Steel: sandblasted 5692 22.8 CFsatis. (after 3 h 80° C. - storage) Standard deviation: ±374.3 ±1.5 ABS:smooth 2254 9.0 MF (after 3 h 80° C. - storage) Standard deviation:±115.0 ±0.5 PVC: smooth 1596 6.4 MF (after 3 h 80° C. - storage)Standard deviation: ±250.8 ±1.0 PC: Smooth 1854 7.4 MF (after 3 h 80°C. - storage) Standard deviation: ±107.1 ±0.4

From these experimental results it is clear that the color changecorrelates unambiguously with the required oxygen uptake of the aerobicadhesive.

Similarly, with the dyes bromophenol blue and 5-nitroindazolinone, therewas an unambiguous correlation of the clearly visible color change withthe required oxygen uptake of the aerobic adhesive composition.

What is claimed is:
 1. An aerobically curable composition based onfree-radically polymerizable compounds, which comprises an activatorsystem for forming peroxides in situ upon contact with air or oxygen anda dye as an indicator of the uptake of oxygen whose color change inaqueous solution lies within a pH range from 3.0 to 6.5.
 2. Acomposition as claimed in claim 1, wherein said free-radicallypolymerizable compounds comprise A) at least one polyurethane(meth)acrylate of the general formula[H₂C═CR¹—C(═O)—O—R²—O—C(═O)—NH—]_(n)R³  (I)  in which R¹=hydrogen or amethyl group R²=a linear or branched alkyl group having 2 to 6 carbonatoms or alkylene oxides having 4 to 21 carbon atoms, n=2 or 3, and R³if n=2 is: [—Q—NH—C(═O)]₂[{—O—R⁴—O—C(═O)—NH—Q′—NH—C(═O)}_(m)—O—R⁴—O—] in which m=0 to 10, R⁴ is a) a polycaprolactonediol radical, b) apolytetrahydrofurfuryldiol radical or c) a diol radical derived from apolyester diol characterized by a C:O ratio of >2.6, a C:H ratio of <10,and a molecular weight from 1000 to 20,000, or d) a diol radical derivedfrom a polyester-diol which is liquid at 20° C. and has a molecularweight of from 4000 to 10,000, measured with the aid of GPC (gelpermeation chromatography), and R³ if n=3 is:[—Q—NH—C(═O)—O—((CH₂)₅—C(═O))_(p)—]₃R⁵  in which R⁵ is a triol radicalof a linear or branched trivalent alcohol containing 3 to 6 carbon atomsand p is from 1 to 10, and Q and Q′, independently of one another, arearomatic, aliphatic or cycloaliphatic groups which contain 6 to 18carbon atoms and are derived from diisocyanates or mixtures ofdiisocyanates, and also B) one or more (meth)acrylate comonomers,wherein said composition comprises (a) 20-80% by weight of thepolyurethane (meth)acrylate, and (b) 80-20% by weight of the(meth)acrylate comonomer, based on the overall amount of polymerizablecompounds, and the activator system is a hydrazone activator system. 3.A composition according to claim 2, comprising (a) 20-70% by weight ofthe polyurethane (meth)acrylate and (b) 80-30% by weight of the(meth)acrylate comonomer, based in each case on the overall amount ofpolymerizable compounds.
 4. A composition as claimed in claim 1, whereinR² is selected from ethylene, propylene, isopropylene, n-butylene orisobutylene, ethylene oxide or propylene oxide units.
 5. A compositionas claimed in claim 1, wherein Q and Q′ independently of one another arederived from diisocyanates selected from 2,4-toluene diisocyanate,2,6-toluene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI),4,4′-dicyclohexyldiisocyanate, meta- and para-tetramethylxylenediisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate(isophorone diisocyanate), hexamethylene diisocyanate, 1,5-naphthylenediisocyanate, dianisidine diisocyanate,di(2-isocyanatoethyl)bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate, 2,2,4-and 2,4,4-trimethylhexamethylene diisocyanate, and mixtures thereof. 6.A composition as claimed in claim 1, wherein the (meth)acrylatecomonomer (B) is selected from allyl acrylate, allyl methacrylate,methyl acrylate, methyl methacrylate, 2-hydroxyethyl acrylate,2-hydroxyethyl methacrylate, 2- or 3-hydroxypropyl acrylate, 2- or3-hydroxypropyl methacrylate, 6-hydroxyhexyl acrylate, 6-hydroxyhexylmethacrylate, benzoyl methacrylate, phenylethyl methacrylate,2-phenoxyethyl methacrylate, morpholinoethyl methacrylate, glycidylmethacrylate, piperidylacrylamide, neopentyl methacrylate, cyclohexylmethacrylate, tert-butyl methacrylate, and tetrahydrofurfurylmethacrylate, or mixtures thereof, and also dimethacrylate.
 7. Acomposition as claimed in claim 1, wherein the activator systemcomprises hydrazone compounds of the general formula (II)

in which the radicals R⁶, R⁷ and R⁸ are at least partially identical ordifferent and have the following meanings: R⁶ is a straight-chain,branched or cyclic and optionally substituted alkyl radical or anoptionally substituted aryl radical, R⁷ is hydrogen, a straight-chain,branched or cyclic and optionally substituted alkyl radical, or anoptionally substituted aryl radical, or R⁶ and R⁷, together with theshared substituted carbon atom, to form a cycloaliphatic radical, whichmay also be substituted; R⁸ is a straight-chain, branched or cyclic andoptionally substituted alkyl radical or an optionally substituted arylradical.
 8. A composition as claimed in claim 7, further comprisingmetal compounds at least partially soluble in the system, wherein saidmetals are selected form a group consisting of iron, copper, cobalt andmanganese.
 9. A composition as claimed in claim 7, further comprisingco-components with reduction potential from redox systems which initiatefree-radical polymerizations are used as accelerators.
 10. A compositionas claimed in claim 7, wherein the components of the activator systemare used in the following concentrations by weight, based in each caseon the overall mixture: (a) hydrazone compounds, from 0.1% to 7.5%, (b)metal compounds, from 50 ppm to 1%; (d) reducing agents, from 1 to 5%.11. A composition as claimed in claim 7, wherein alpha-hydroxy ketonesare used as reducing agents having an accelerator action.
 12. Acomposition as claimed in claim 1, wherein the dye color change takesplace from colorless/yellowish to red/blue, green or to thecomplementary colors of red, blue, green, and yellow.
 13. A compositionas claimed in claim 1, wherein said dye is selected from the groupconsisting of 5-nitroindazolinone, bromophenol blue, bromochlorophenolblue, bromocresol green, alizarin S, methyl red, and ethyl red.
 14. Acomposition as claimed in claim 1 in spreadable form.
 15. A method fordetermining the sufficiency of oxygen uptake in the adhesive bonding ofmaterials with an aerobically curing reactive adhesive composition, saidmethod comprising (a) applying to at least one of the surfaces of thematerials that are to be connected to one another an aerobically curableadhesive composition, said adhesive composition comprising (i) at leastone free-radically polymerizable compound, (ii) an activator system forforming peroxides upon contact with air or oxygen, and (iii) a dye as anindicator of the uptake of oxygen whose color change in aqueous solutionlies within a pH range from 3.0 to 6.5; (b) heating the materials on thesurface provided with the adhesive composition to a temperature of20-80° C. during the contact of the adhesive composition in the presenceof atmospheric oxygen (c) monitoring the uptake of oxygen by monitoringthe color of the dye indicator of the heated adhesive composition, (d)after the heated adhesive composition has taken up a sufficient amountof oxygen, as indicated by the appropriate color change of the dyeindicator, joining the materials to one another at the desired surfaces,and (e) curing the adhesive composition.
 16. A method as claimed inclaim 15, wherein the dye indicator color change takes place fromcolorless/yellowish to red/blue, green or to the complementary colors ofred, blue, green, and yellow.
 17. A method as claimed in claim 15,wherein said dye is selected from the group consisting of5-nitroindazolinone, bromophenol blue, bromochlorophenol blue,bromocresol green, alizarin S, methyl red, and ethyl red.
 18. A methodaccording to claim 15, wherein said free-radically polymerizablecompounds comprise A) at least one polyurethane (meth)acrylate of thegeneral formula [H₂C═CR¹—C(═O)—O—R²—O—C(═O)—NH—]_(n)R³  (I)  in whichR¹=hydrogen or a methyl group R²=a linear or branched alkyl group having2 to 6 carbon atoms or alkylene oxides having 4 to 21 carbon atoms, n=2or 3, and R³ if n=2 is:[—Q—NH—C(═O)]₂[{—O—R⁴—O—C(═O)—NH—Q′—NH—C(═O)}_(m)—O—R⁴—O—]  in which m=0to 10, R⁴ is a) a polycaprolactonediol radical, b) apolytetrahydrofurfuryldiol radical or c) a diol radical derived from apolyester diol characterized by a C:O ratio of >2.6, a C:H ratio of <10,and a molecular weight from 1000 to 20,000, or d) a diol radical derivedfrom a polyester-diol which is liquid at 20° C. and has a molecularweight of from 4000 to 10,000, measured with the aid of GPC (gelpermeation chromatography), and R³ if n=3 is:[—Q—NH—C(═O)—O—((CH₂)₅—C(═O))_(p)—]₃R⁵  in which R⁵ is a triol radicalof a linear or branched trivalent alcohol containing 3 to 6 carbon atomsand p is from 1 to 10, and Q and Q′, independently of one another, arearomatic, aliphatic or cycloaliphatic groups which contain 6 to 18carbon atoms and are derived from diisocyanates or mixtures ofdiisocyanates, and also B) one or more (meth)acrylate comonomers,wherein said composition comprises (a) 20-80% by weight of thepolyurethane (meth)acrylate, and (b) 80-20% by weight of the(meth)acrylate comonomer, based on the overall amount of polymerizablecompounds, and the activator system is a hydrazone activator system. 19.A method according to claim 18, wherein the composition comprises (a)20-70% by weight of the polyurethane (meth)acrylate and (b) 80-30% byweight of the (meth)acrylate comonomer, based in each case on theoverall amount of polymerizable compounds.
 20. A method according toclaim 18, wherein the (meth)acrylate comonomer (B) is selected fromallyl acrylate, allyl methacrylate, methyl acrylate, methylmethacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 2-or 3-hydroxypropyl acrylate, 2- or 3-hydroxypropyl methacrylate,6-hydroxyhexyl acrylate, 6-hydroxyhexyl methacrylate, benzoylmethacrylate, phenylethyl methacrylate, 2-phenoxyethyl methacrylate,morpholinoethyl methacrylate, glycidyl methacrylate,piperidylacrylamide, neopentyl methacrylate, cyclohexyl methacrylate,tert-butyl methacrylate, and tetrahydrofurfuryl methacrylate, ormixtures thereof, and also dimethacrylate.
 21. A method according toclaim 15, wherein the activator system comprises hydrazone compounds ofthe general formula (II)

in which the radicals R⁶, R⁷ and R⁸ are at least partially identical ordifferent and have the following meanings: R⁶ is a straight-chain,branched or cyclic and optionally substituted alkyl radical or anoptionally substituted aryl radical, R⁷ is hydrogen, a straight-chain,branched or cyclic and optionally substituted alkyl radical, or anoptionally substituted aryl radical, or R⁶ and R⁷, together with theshared substituted carbon atom, to form a cycloaliphatic radical, whichmay also be substituted; R⁸ is a straight-chain, branched or cyclic andoptionally substituted alkyl radical or an optionally substituted arylradical.
 22. A method according to claim 21, wherein said activatorsystem further comprises metal compounds at least partially soluble inthe system, wherein said metals are selected from a group consisting ofiron, copper, cobalt and manganese.
 23. A method according to claim 7,said activator system further comprises co-components with reductionpotential from redox systems which initiate free-radical polymerizationsare used as accelerators.
 24. A method according to claim 23, whereinthe components of the activator system are used in the followingconcentrations by weight, based in each case on the overall mixture: (a)hydrazone compounds, from 0.1% to 7.5%, (b) metal compounds, from 50 ppmto 1%; (c) reducing agents, from 1 to 5%.
 25. A method according toclaim 23, wherein alpha-hydroxy ketones are used as reducing agentshaving an accelerator action.
 26. A method according to claim 15 whereinthe aerobically curable adhesive composition is applied as a spreadablesurface coating, molding compound, or additive to inks, includingprinting inks.
 27. A method according to claim 15 wherein the materialsto be bonded are identical or different materials selected from metal,plastics, ceramic, glass and cellulosic materials.
 28. A methodaccording to claim 15 wherein the materials to be bonded compriseloudspeaker components.